1. Technical Field
The present disclosure relates to resonators involving varactors, and more particularly to wideband temperature compensated resonators involving varactors such as resonators usable in wideband Voltage Controlled Oscillators (VCOs).
2. Background Information
Resonators are used in many different types of electronic circuits. One type of resonator involves an inductive element coupled in parallel with a capacitive element. An application of such a resonator is a Voltage Controlled Oscillator (VCO) such as a VCO found in a Phase-Locked Loop (PLL). FIG. 1 (Prior Art) is a diagram of one such type of VCO 31. VCO 301 generates an oscillating VCO output signal. In the illustrated example, the oscillating VCO output signal is a differential sinusoidal signal involving signal VOUT− on conductor 302 and VOUT+ on conductor 303. The frequency of the oscillating VCO output signal is determined by a multi-bit digital coarse tune control word received on conductors 304 as well as a fine tune analog control signal VTUNE received on conductor 305. Assuming that the digital control word is fixed, the frequency of the oscillating VCO output signal can be fine tuned up and down by appropriately increasing or decreasing the analog input signal VTUNE.
FIG. 2 (Prior Art) is a more detailed diagram of one example of such a VCO. The resonator tank 306 involves an inductor 307 coupled in parallel with capacitive elements. One of the capacitive elements is a programmable capacitor bank 308. A second of the capacitive elements is a main varactor circuit 309. In one example, the main varactor circuit 309 is a programmable varactor that involves multiple varactor sub-circuits. Individual ones of the varactor sub-circuits can be disabled to decrease the effective tunable capacitance of the programmable varactor element. See U.S. Pat. No. 7,612,626 for additional information on this type of programmable varactor. The remaining transistors 310-313 of the VCO of FIG. 2 form an amplifier. If the digital control word supplied to the VCO is fixed, and if the VTUNE analog input voltage as supplied to the VCO is fixed, then it is desired that the open loop oscillating frequency of the VCO output signal VOUT+, VOUT− be a fixed frequency. Unfortunately, the open loop oscillating frequency of the VCO output signal is seen to change with temperature. The oscillating frequency may, for example, drop as temperature increases. This is undesirable.
FIG. 3 (Prior Art) is a circuit diagram of one conventional VCO circuit 314 that has circuitry for removing the temperature dependent nature of changes in the open loop oscillating frequency of the VCO output signal of the VCO. In addition to the main varactor 315 and the coarse tuning capacitor bank 316, an auxiliary varactor 317 is provided. For additional detail on such an auxiliary varactor, see U.S. Patent Application Publication US2009/0261917. Rather than auxiliary varactor 317 receiving the VTUNE signal such that the capacitance of the auxiliary varactor is adjusted as a function of VTUNE, the auxiliary varactor is made to receive an analog control voltage VCOMP instead. VCOMP is a control voltage generated by a temperature compensating voltage generating circuit 318. VCOMP is made to change as a function of temperature so that the resulting changes in the capacitance of the auxiliary varactor 317 tend to counter the other temperature dependent effects of the remainder of the VCO on VCO output signal frequency. As a result, the open-loop frequency drift of the oscillating frequency of the VCO as a function of temperature can be reduced.
FIG. 4 (Prior Art) is a circuit diagram of another conventional VCO circuit 319 that has circuitry for preventing frequency drift of the oscillating frequency of the VCO. The coarse tuning capacitor bank 320 involves multiple portions 321-323. The circuitry an individual portion of the capacitor bank can be enabled to switch in a capacitance in parallel with inductor 324, or the circuitry of the individual portion can be disabled so that the capacitance is not coupled in parallel with the inductor. Circuitry is provided so that in a disabled condition, the degree of reverse biasing of parasitic diodes of the disabled portion can be adjusted, thereby allowing the capacitance of the disabled portion to be adjusted as a function of temperature. By proper adjusting of the analog control voltage VCOMP supplied to disabled portions, the open-loop frequency drift of the VCO as a function of temperature can be reduced. For additional detail, see U.S. Pat. No. 7,116,183. The amount of temperature compensation afforded is a function of the number of portions of the capacitor bank that are disabled at a given time. At low frequencies, when all the portions 321-323 of the capacitor bank are used, there is no temperature compensation.